The present invention relates to an X-ray image intensifier for intensifying and reproducing an image of an object, by use of radiation such as X-rays, gamma rays, and the like and, more particularly, to an X-ray image intensifier having an improved evacuated envelope.
In the field of medicine, X-ray image intensifiers are widely used for medical diagnosis, and in the industrial field, on the other hand, they are used in nondestructive testing.
In general, an X-ray image intensifier comprises an input screen, an output screen, a focusing electrode, an anode, and the like, and an evacuated envelope within which these components, arranged at appropriate positions therewithin, are sealed. More specifically, the input screen is arranged at the X-ray input side, and the output screen and the anode are arranged at the X-ray output side. The two screens face each other. The input screen is shaped as a focusing lens constituting an aspheric surface so as to form a focal plane on the output screen. The focusing electrode is arranged along the inner wall of the evacuated envelope, and guides photo electrons radiated from the input screen toward the output screen, while simultaneously accelerating and focusing them.
The evacuated envelope comprises a cylindrical body having one open and closed end, with a dome-like X-ray input window covering the open end of the body. The cylindrical body is formed of a glass or ceramic insulating material, in order to prevent discharging of an acceleration electrode and also to output a visible optical image or an image intensifying signal such as an electrical signal converted inside the evacuated envelope. The X-ray input window is fitted on the input side of the evacuated envelope, and is in the form of a metal plate made of either Al (aluminum) or Al alloy in order to lower an amount of scattered incident X-rays and to maintain a mechanical strength of the window at a predetermined level or higher. The X-ray incident side of the window is shaped as an aspheric surface, in the same manner as the input screen, so as to uniformly radiate X-rays onto the input screen in the evacuated envelope.
When the image intensifier having the above structure is in operation, a predetermined potential difference is provided across the focusing electrode and the anode. X-rays radiated from an X-ray source are transmitted through the subject being examined and the input window, and thereafter are incident on the input screen. The incident X-rays are converted to visible light by a phosphor layer formed by coating a phosphor material, e.g., CsI on the input screen, and the visible light is converted to photo-electrons by a photoemissive layer formed thereon. The photo-electrons are accelerated and focused by the focusing electrode and the anode constituting an electron lens, and are again converted to visible light by the output screen arranged behind the anode. As a result, a visible image is formed on the output screen.
The image can be directly observed through the output window, or through a television set, or can be visualized as a photograph. Using the image, medical diagnosis can be performed.
At the X-ray input side of the image intensifier, X-rays are not radiated directly onto the input screen having X-ray-visible light conversion and photoelectric conversion functions, but are instead radiated indirectly thereonto, through the input window.
The interior of the evacuated envelope must be kept at a high vacuum level, of 10.sup.-7 to 10.sup.-8 Torr, in order to stably pass photo-electrons from the input screen toward the output screen. If the window is omitted and the input screen is directly used to act as a window to maintain the high vacuum, the input screen is deformed or cracked by the outside atmospheric pressure directly applied thereonto. As a result, predetermined photoelectrons cannot be emitted from the input screen in a predetermined manner.
Since the interior of the evacuated envelope is maintained at a high vacuum, if the structure and material of the input window are inappropriately selected, the input window may be broken by the outside atmospheric pressure, i.e., a so-called implosion may occur. The implosion may be suppressed by increasing the thickness of the input window to strengthen the window.
However, increasing the thickness of the input window will result in a decrease in the SN ratio (signal-to-noise ratio) of incident X-rays as well as an undesirable increase in the total weight of the overall image intensifier. For this reason, Al or an Al alloy is used as the material of the input window, since it has a high strength-to-weight ratio and good X-ray transmissivity, and minimizes X-ray scattering.
However, it is not easy to achieve a hermetic bond between an Al material and a ceramic material or glass. This obstructs a manufacture of a large image intensifier.
A technique has been proposed wherein an intermediate member (support ring) that can be stably and hermetic bonded to a glass or ceramic material is provided between a glass body and an aluminum window, and these members are bonded through the intermediate member.
West German Patent No. 2331210 is known as a technique for hermetic bonding an envelope body and an input window through a support ring. In this known technique, a peripheral portion of an aluminum window is bent to extend along the inner wall of a cylindrical body, a copper support ring is interposed between the body and the window, and the window is bonded to the body through the support ring to be fitted therein.
However, in this known technique, since the end portion of the support ring is extended by bonding and projects from the evacuated envelope, the projecting portion must be removed after bonding, resulting in complex machining and high manufacturing cost.
As another technique for hermetic bonding of a body and an input window through a support ring, U.S. Pat. No. 4,423,351 is known.
According to FIG. 4 in U.S. Pat. No. 4,423,351, input window 24, intermediate metal ring 42 and washer ring 50 are coaxially provided. Peripheral portion 41 of window 24 is overlaid to be clamped between a joint of ring 42 and washer ring 50, and the overlapping portion of these three components is subjected to hot press welding by ring tip portions 48a and 49a of press welding apparatuses. The welded portion is flat, and extends to be perpendicular to the axis of a vacuum container (evacuated envelope). In order to ensure the mechanical strength of the support portion, the thickness of intermediate metal ring 42 is larger than that of input window 24. Peripheral portion 41 and the joint portion of intermediate metal ring 42 extend in a direction perpendicular to the axis of the vacuum container, and finally, the inner end of peripheral portion 41 is at the same position a that of the joint portion of intermediate metal ring 42, as shown in the drawing. More specifically, the inner diameters of intermediate metal ring 42 and peripheral portion 41 are designed to coincide with each other. In order to improve a bonding property between intermediate metal ring 42 and a glass body (not shown), an Fe or Fe based alloy (to be referred to simply as an Fe material hereinafter) such as an Fe--Ni--Co alloy called Kovar (trade name) is employed as a material of intermediate metal ring 42. Note that in order to improve a bonding property between intermediate metal ring 42 and aluminum window 24, thin Ni plated layer 45 is formed on the surface of ring 42.
However, in the conventional image intensifier, if its interior is evacuated, a curved portion formed on a transitional area from the domed portion of input window 24 to the peripheral portion 41 is acted upon an atmospheric pressure and is recessed inwardly, and finally, input window 24 may be imploded.